It is well known that when a pulverized metal oxide, such as hematite, is mixed with pulverized carbon and the mixture is heated, the oxide will be reduced to metal and, if the temperature is high enough, the metal and associated gangue will be melted to form molten metal and slag. Known processes perform such heat treatment in shaft furnaces fueled by the combustion of carbon added as raw coal or as lump coke.
In the former case, for example, Worner, U.S. Pat. No. 3,652,069 issued March 1972, the metal oxide is mixed with raw coal in excess of that necessary for reduction, and the volatile matter in the coal is driven off in the upper part of the shaft to form a low-temperature coke. The excess coke descends with the charge to tuyere level near the base of the shaft where it is burned with cold or hot air. In the latter case, as in Obenchain, U.S. Pat. No. 3,832,158 issued Aug. 27, 1974, pellets made of a mixture of metal oxide and carbon are charged in layers alternating with layers of lump coke, the coke being burned near tuyere level with cold or hot air.
The difficulties with the use of a raw coal as in Worner are in controlling the carbonization or devolatilization in the upper part of the stack, in condensation of tar in the top of the furnace and in gas offtakes, and the fact that the coke so formed is a low temperature variety which burns at low combustion efficiency and resulting high consumption of carbon. A coking coal is also necessary. The difficulties with the case of lump coke as specified by Obenchain are the necessity to use expensive lump coke as fuel, the layered type of charge distribution, and lack of definition of conditions of combustion except to describe the combination as utilizing "air through tuyeres as in a blast furnace".
The most important part of such processes is the combustion of carbon to provide heat, which should be done at maximum combustion efficiency, i.e. maximum content of carbon dioxide in final products of combustion, and should use low cost forms of carbon. This requires the specification of both the conditions of combustion and of the type or nature of the carbon being burned, but this is not done in either of the patents quoted nor in other known patents involving combustion in solid carbon fueled shaft furnaces.